Abstract
The study of the reaction between plutonium and nitrogen is helpful in further understanding the interaction between plutonium and air molecules. Currently, there is no research on the microscopic reaction mechanism of plutonium nitridation reactions. Therefore, the microscopic mechanism of the Pu with N2 gas phase reaction is explored in this study, based on density functional theory (DFT) using different basis functions. In this paper, the geometry of stationary points on the potential energy surface is optimized. In addition, the transition states are verified by frequency analysis and intrinsic reaction coordination (IRC). Finally, we obtained the reaction potential energy curve and micro reaction pathways. Analysis of the reaction mechanism shows that the reaction of Pu with N2 has two pathways. Pathway 1 (Pu + N2 → R1 → TS1 → PuN2) has a T-shaped transition state and pathway 2 (Pu + N2 → R2 → TS2 → PuN + N) has an L-shaped transition state. Both transition states have only one imaginary frequency. According to the comparison of the energy at each stagnation point along the two pathways, and the heat energy emitted by the two reaction paths, we found that pathway 1 is the main reaction pathway. The nature of Pu-N bonding evolution along the pathways was studied by atoms in molecules (AIM) and electron localization function (ELF) topological approaches. In order to analyze the role of the plutonium atom 5f orbital in the reaction, the variation in density state along the pathways was measured. Results show that the 5f orbital mainly contributes to the formation of Pu-N bonds, and the influence of temperature on the reaction rate is revealed by calculating the rate constants of the two reaction pathways.
Highlights
The gas phase of actinide elements study can reveal a lot of reaction details which are difficult to obtain by experiments
The gas phase reaction of plutonium and nitrogen is studied by density functional theory (DFT) method
The results show the microscopic reaction mechanism of this reaction in detail
Summary
The gas phase of actinide elements study can reveal a lot of reaction details which are difficult to obtain by experiments. There are scholars have done quantum chemical calculations on the detailed reaction mechanism of gas actinides atoms and gas small molecules, including Pu+O2 [11], U+(U2+)+H2O [12], Pu+H2O [13], etc They obtained multiple reaction pathways of these reactions and the geometric structures of each stationary point along the reaction pathway. By calculating the partial DOS (PDOS) of 5f orbital of each equilibrium and transition state structure and comparing them with the total DOS (TDOS) of the structure and the overlap population DOS (OPDOS) of the interaction between plutonium atom and nitrogen atoms, the contribution of 5f orbital of plutonium atoms to the bond evolution process can be obtained. It is worth emphasizing that the thermodynamic data needed for calculation are obtained by Shermo program [39]
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